Let suppose I have created one record . On this record there is lookup to user. whichever user I will select on this record that user able to see the record when he login in salesforce. How to acheive this.
Assuming the OWD of the object is
Private for which the record is getting shared.
Since the record will be shared to the user when lookup (User) is selected, so you need to create Apex managed sharing for this record to be shared to user during saving this record.